The present invention relates to a composition and method for inhibiting lime scale formation and chloride-induced corrosion of ferrous metals and alloys in processes in which aqueous solutions are concentrated, thus increasing the concentration of ionic species, even in water of high hardness and high chloride content. As used in the present specification, "aqueous solution" refers to ion-containing solutions which are primarily composed of water, such as tapwater or saltwater and the like.
Significant problems exist for systems in which aqueous solutions become concentrated, such as flash heating systems in which feedwater is vaporized by contact with a heat transfer surface. Examples of such flash heating systems include the food storage cabinets used by the fast food industry, in which water is flash evaporated by contact with a metal heat transfer surface to provide a hot, humid environment for temporary food storage. Problems associated with these systems include chloride-induced corrosion and stress-corrosion cracking in systems constructed of ferrous metals and alloys, such as austenitic stainless steels and carbon steels.
Austenitic stainless steels and the like are generally resistant to the corrosive action of most environments due to the formation of a passive oxide film on the surface of the metal. Halides in general and chlorides in particular, however, readily promote localized pitting corrosion of austenitic steels by penetrating this passive oxide film at localized sites, which in turn further breaks down the surrounding passive areas. This results in more corrosive attack by the chloride containing water.
Pitting, and the concomitant corrosion fatigue, often referred to as stress corrosion cracking, can occur in systems using water having only nominal chloride concentrations, such as 10 parts per million or less. This is a particular problem in flash heating systems, as the instantaneous vaporization of the feedwater on the heat transfer surface tends to concentrate the chloride ions on that surface and thus creates localized areas of high chloride concentration.
An additional factor contributing to the problems of pitting and stress corrosion cracking is formation of limescale deposits on the heat transfer surface. These deposits typically consist of 90-95% calcium carbonate (CaCO.sub.3) and 5-10% of magnesium carbonate (MgCO.sub.3). The solubility of these calcium and magnesium carbonate salts in the feedwater decreases as the temperature of the feedwater increases. Consequently, flash evaporation of even low hardness water can cause significant limescale deposition on heat transfer surfaces.
Limescale deposits contribute to localized pitting and corrosion by preventing oxygen from contacting the heat transfer surface. This prevents the formation and maintenance of the protective passive oxide film that usually forms on austenitic surfaces. Limescale formation also acts as a barrier to heat transfer, which in turn means that the use of higher temperatures to achieve proper flash vaporization is required. These higher temperatures increase the magnitude of the stress cycle created by the constant heating and cooling of the heat transfer surface, thus further promoting stress corrosion cracking.
In order to overcome these problems, one approach has been pretreatment or conditioning of the feedwater in an effort to reduce the concentrations of deleterious ions. Several methods have been developed to accomplish this reduction, including the use of reverse-osmosis and ion-exchange filtration units. These methods, however, while providing significant reductions in the ion content of the feedwater, cannot completely remove all of the deleterious ions from the feedwater. Consequently, these methods slow development of corrosion and scale but cannot eliminate their formation.
An alternative approach to reducing corrosion involves the addition of corrosion inhibitors to the feedwater. These corrosion inhibitors are generally classified according to which half of the corrosion reaction they inhibit.
More specifically, anodic inhibitors prevent the migration of metal ions from the metal surface into the water, thus inhibiting the anodic half of the reaction. Anodic inhibitors typically act by initiating or promoting the formation of an oxide film on the metal heat transfer surface. These inhibitors, however, are usually quite sensitive to the pH and chloride ion concentration of the feedwater, and generally have a critical minimum concentration, below which the inhibitor will actually facilitate localized corrosion. Because of this problem, anodic inhibitors are generally not used to prevent this type of corrosion. Examples of typical anodic inhibitors include chromates, nitrates and nitrites, molybdates and phosphates. These inhibitors may be used individually or in combination with one another or even together with cathodic inhibitors.
Unlike anodic inhibitors, cathodic inhibitors isolate the cathodic area of the metal surface from the aqueous environment by precipitating and forming an insoluble film, thus inhibiting the cathodic half of the reaction by preventing further reaction at these sites. Cathodic inhibitors are generally less effective corrosion inhibitors than anodic inhibitors. Examples of typical cathodic inhibitors include zinc ions, polyphosphates and phosphonates. Cathodic inhibitors may be used individually or in combination with other cathodic and/or anodic inhibitors.
Limescale formation on heat transfer surfaces may also be controlled by chemical treatment of the feedwater. In the past, such treatment usually involved addition of acidic compounds, such as sulfuric acid, to the feedwater, causing a shift in the solubility equilibrium away from formation of calcium carbonate.
The more common treatments in use today tend to focus on inhibiting either the nucleation or crystal growth stages of the scale formation process. For example, poly(inorganic phosphates) and phosphonates adsorb onto nuclei and growing limescale crystals, thus inhibiting both nucleation and crystal growth. Under certain conditions, however, the polyphosphates can hydrolyze to form phosphate ions, which both destroys the inhibiting action of the additive and actually encourages the formation of insoluble calcium phosphate scale. Similarly, phosphonates are degraded to phosphate ions by aqueous solutions containing even low concentrations of chloride.
Organic polymers are also presently used to control scale formation. These polymers, such as polyacrylates and polymethacrylates, act as dispersants and prevent scale formation by keeping small particles of scale in suspension. Organic polymers, however, are generally not suitable for use in hot aqueous environments, such as flash heating systems, where they would degrade and decompose.